Low cost controller



May 8, 1962 J. STRATTON LOW COST CONTROLLER Filed June 15, 1959 funk. If" v i m 0 5 .0 6 A .0 E aw R B :4. wwllmllwll I W A A F A I? I |lll C w QQQ C0//ect0r- Current Co/lecl'lor Current Inventor": Jerry L. Stratton, y H/s Attorney Ma Ft My invention relates to transistor circuits, and particularly to such circuits adapted for use in effecting desired control operations such as the operation of relays, etc.

It has for one of its objects to provide an improved circuit having extremely sensitive response to minute voltage variations to produce a current variation sutficient for reliable actuation of the desired control apparatus.

A further object of my invention is to provide such a circuit which is simple and economic to construct and operate, which utilizes a minimum of energy under standby conditions, but which, in response to a slight voltage variation, which may be produced as the result of variation of a primary variable such as temperature, light, etc., reliably produces variations to operate control equipment that compensates for or restores the primary variable to its orginal condition, or to eiiect other desired control functions.

A further object of my invention is to efiect such results by the use of a single amplifying device such as a transister, and to avoid the use of vacuum tubes, saturable reactors, or other costly components.

I employ a transistor in a regenerative circuit having a relaxation characteristic and, by varying the gain of the transistor in response to the minute controlling voltage variation, the circuit may be varied from an oscillating condition having small effective collector current to a non-oscillating condition having a large collector current. Thus, a large collector current variation ample for most control operations may be obtained inresponse to very minute voltage variations applied to the base.

My invention, however, is not limited to construction with a transistor and, as will appear hereinafter, other types of amplifying devices may be utilized. in its preferred form, however, a single transistor amplifying device is employed in the circuit.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization andmethod of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

FIGURE 1 represents an embodiment of my invention;

FIGURE 2 represents the gain vs. collector current characteristic thereof;

I FIGURE 3 illustrates the collector current during oscillation; and

FIGURE 4- represents a modification of the circuit shown in FEGURE 1 employing the condition sensitive control element as a part or the feedback circuit.

Referring now to FIGURE 1 of the drawing, 1 have indicated at 1 by the conventional symbol for transistors, a transistor comprising a collector 2, an emitter 3, and a base 4. In the circuit here illustrated, this transistor is preferably one which, during operation, carries approximately equal current in both the emitter and collector and only the difference in currents in these electrodes flows in the base Such a transistor may be one of the 2N43 type which employs a slab of semiconductor having on its opposite sides and partially embedded therein the two electrodes designated collector and emitter, each of them having areas and substantial current carrying capacity.

Unidirectional operating potential is supplied to the transistor from any suitable source, but which in the 3,fl34,d23 Patented May 8, 162

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drawing is shown as comprising a full wave rectifier 11, comprising a transformer 12, the primary of which may be connected to any suitable alternating current source, and the secondary of which has its opposite terminals connected through unilaterally conducting devices 13 and 14 to the negative conductor 15, whereas the midpoint of the transformer is connected to a positive conductor 16. A suitable smoothing and filtering capacitor 17 is connected between these conductors. The negative conductor 15 is connected through the winding 18 of a suitable relay 19 to the collector 2 of the transistor.

The emitter 3 and base 4 of the transistor are connected to diagonally opposite points 24 and 25 of a Wheatstone bridge, one branch of which comprises resistances 26 and 27 and the other of which comprises resistances 28, 29, and 30.

Resistance 30 may be variable in response to the primary variable which it is desired to control, which may be pressure, humidity, temperature, or other quantity. In the event that it is desired to employ the equipment to control the temperature of a room, for example, as in household heating systems, the resistance 30 may be a thermistor. These devices have a negative temperature co-efiicient such that the resistance of the thermistor decreases as the temperature increases. Resistance 28 in the branch 28, 29, and 30 may be manually variable by movement of contact 32 thereon, thereby efiectively short circuiting a part of the resistance. In this way, the temperature to be maintained by the equipment may be manually adjusted at any desired temperature in a range of temperatures.

The diilerence in potential between points 24 and 25 is applied through resistance 33 across the transistor emitter and base electrodes 3 and 4, thereby to control the operation of the transistor. The bridge is so adjusted that point 25 is normally negative with respect to point 24, and base 4 is negative with respect to emitter 3, by an amount dependent on the value of resistance 30 which varies with the temperature to be controlled.

The collector 2 of the transistor is connected through relay Winding it? to the negative terminal of the source. This winding i8 is inductively coupled to a second winding 34, which may be on the relay, and which is shunted by the series combination of resistances 35 and 36, the lower terminal of resistance '36, as shown on the drawings, being connected through a capacitor 37 to the base electrode 4, and through resistance 33 to point 25 of the bridge. Resistance 35 is normally small, as for example 500 ohms, relative to the value of resistance 36, which may be 4,000 ohms, and it has a variable contact 38 which is connected to the emitter 3 and to point 24 on the bridge, which is at a relatively fixed potential. Variation of contact 38 on resistance 35 varies to some extent the value of the loop gain including the transistor, windings 13 and 34 and coupling therebetween together with the connections described from winding 34 back to the transistor. This results in a variation of the value of collector current at which oscillations start. I

Variation in current in winding 18 induces a voltage in Winding 3 which, since contact 38 is at fixed potential, is supplied back through condenser 37 to base 4 to support oscillations as will presently be described.

Condenser 37 normally is charged to a certain extent with its upper electrode, as shown on the drawing, positive with respect to the lower electrode. The upper electrode is connected through resistances 36 and 35 and contact 38 to point 24 which normally is positive with respect to point 25 to which the lower electrode is con nected. This charge on condenser 37 is increased by any potential appearing on resistance 33 due to current flowing therein.

The gain of the oscillatory loop comprising the transistor, windings 18 and S4, and the coupling therebetween, and the coupling from winding 3 3 back to the base 4, varies, due to variation in gain of the transistor, with the current flowing in the collector in accordance with the relationship illustrated in FIGURE 2, in which the full line curve bearing the legend Loop Gain represents the relationship between gain plotted as ordinate against collector current plotted as abscissa. It will be seen that this curve, over the larger portion of its range, is relatively flat but varies gradually downward with increasing collector current.

The horizontal dash line bearing the legend Loop Gain=l represents the value of loop gain equal to unity. In the event that the gain be greater than unity, as in area A of FIGURE 2 left of the vertical dash line, the circuit described oscillates. In area B, to the right of the vertical dash line, the gain is less than unity and the device is non-oscillatory. Assume the circuit is nonoscillating and the value of collector current is slightly to the right of the vertical dotted line shown in FIGURE 2. Now assume that the base current decreases slightly to a point slightly to the left of this line. This change increases the loop gain and oscillation starts, the nature of which is shown in FIGURE 3. Oscillations will stop any time that the average value of the base current increases enough to cause the peak of the collector current to go above the upper dotted line shown in FIG- URE 3.

Assuming that the temperature to which resistance 30 responds is at a value below that which it is desired to maintain so that the system calls for heat, the resistance of resistor 30, because of the low temperature, is high, and the point 25 is considerably negative with respect to point 24, thereby biasing base 4 negative and in the forward direction with respect to emitter 3 causing large current to flow through the winding 18 and to energize the relay. Because of the large current in the collector and emitter, the gain of the loop is less than unity and the device does not oscillate.

Now, let us assume that because of the energized condition of the coil 18, the relay is operated to such a position that heat is applied, as by starting the furnace, thus raising the temperature of the room. As the temperature of the room approaches the temperature to be maintained, resistance 30 decreases and the potential at point 25' becomes less negative; i.e., approaches the potential of point 24, thereby reducing the bias applied to the base 4 with respect to the emitter and reducing the current in winding 18- until the gain of the loop exceeds comprising the base 4 and emitter 3 of the transistor which is now biased in the forward direction. 7

Finally, the increase in current is limited by the resistance in winding lid, and the increase steps. This causes the voltage on winding 34 to reduce and finally to disappear whereupon the current again starts to decrease in collector 2 reversing the polarity of the potential on winding 34, and the cycle repeats itself.

FIGURE 3 shows the collector current plotted as ordinate against time plotted as abscissa with the origin of the coordinates, in the upper left hand corner and with dash lines A and B representing respectively minimum and maximum collector currents in alignment with corresponding dash lines A and B in FIGURE 2.

Again, reiterating the operation with reference to FIG- URE 3, when the temperature is below the desired value, large current flows in the collector in excess of the value, represented by line B, or the line B of FIGURE 2, and the loop gain is less than unity. The relay is, energized and the furnace is operated, applying heat tending to raise the temperature of the room.

As the temperature increases, the potential on base 4 becomes less negative with respect to that of the emitter 3, and the collector current starts to reduce. When the collector current becomes less than the value represented by, the dotted lines 3 in FIGURE 2, or B in FIGURE 3, the loop gain exceeds unity, regeneration starts and the current drops rapidly to the value repreunity whereupon the system goes into oscillation; i.e.,

the decrease in current in winding 18 induces voltage in winding 34' of polarity such that its lower terminal, which is connected to condenser 3'7, is negative relative to its upper terminal. Then, since contact 3% is at fixed potential, that portion of the potential of winding 34 which exists between contact 3% and condenser 37 is added to the charge on condenser 37' and drives base 4 in the positive direction to such a point that the potential of base 4 may attain or become more positive than emitter 3.

Thus, the collector current in winding 18 rapidly reduces until it becomes of very small value. Then the potential on winding 34- disappears, but the charge on condenser '37 maintains the potential of base 4 relative to emitter 3 at such a value as to delay any increase in collector current. This charge gradually leaks during an interval determined principally by resistance 33 and current begins to increase in winding 18. This reverses the polarity of potential induced in winding 34 thereby rapidly varying the potential of base 4 in a direction to increase current in winding 18; i.e., driving base 4 negative relative to emitter 3. During this time condenser 37 again charges since the potential from winding as on resistance 36 is added to that between points 2 and 2s across the condenser through the low impedance path sented by line A in FIGURE 2 or A in FIGURE 3. This drop in current in winding 18 induces voltage in winding 34 which added to that on condenser 37 drives base 4 in the positive direction until the base becomes more positive than the emitter. At this point it remains due to the charge of condenser 37. As the charge on condenser 37 leaks off, base 4 becomes more negative and the current begins to increase in winding 1%, thereby inducing a voltage in winding 34 opposed to that and condenser 37 thereby further to increase the collector current. When the collector current again approaches the value X, its further increase is limited by the resistance in winding 18 and the loop gain becomes less than unity and regeneration ceases. The voltage induced in winding 34 disappears and the potential of base 4 again approaches the emitter potential thereby further reducing the collector current.

The effective value of the collector current during oscillation, as represented by 'the curve of FIGURE 3 is, of course, much lower than that when the operation started, with the result that relay winding 18 is sufficiently deenergized to cause its armature to drop to its lower position, thereby stopping the furnace. The frequency of oscillation may be of any desired value and may be sufiiciently high, as for example 2,000 cycles per second, to avoid any chattering of the relay.

It will now be seen that it is desirable that the increase in current during the lower curved portions of the curve of FIGURE 3 be as slow as possible, thereby to prolong the intervals between decrease in current and the next subsequent increase therein relative to the interval between the increase and subsequent decrease in current, thereby to produce a greater reduction in effective value of the current during oscillation relative to that occurring during thenon-operating current.

While the increase in collector current during the initial periods is delayed by the time required for condenser 37 to discharge, no similar delay occurs during the interval when the collector current starts to decrease. The result is that the periods of maximum collector current are substantially shorter than the alternate intervals thereby substantially reducing the efiective collector current.

The rapid response and accuracy of the circuit is thus due to the instantaneous change of oscillator state upon a sensed temperature deviation as well as the rapid action of the switching mechanism upon a change of oscillator state. It should 'be noted here that the transistor in the oscillator circuit may have temperature coeflicient characteristics, which will vary internal transistor impedance upon deviation of the sensed temperature. The temperature eifects of the transistor in the embodiment were found to be negligible but by the proper placement of thermistor 30 in the bridge circuit, any such effects may be utilized to increase the sensitivity of the device.

In an operable device constructed according to the preferred embodiment the circuit elements may have the following values: resistor 33=10,000 ohms, capacitor 37:.005 mfd., transistor 4=2N43 pnp junction type,

- voltage divider 35:500 ohms and resistor 36:4,000

ohms. The switch mechanism including winding 18 and member 41 may comprise a suitable milli-ampere relay of medium sensitivity, and secondary Winding 3 may have a turns ratio of about five percent in respect to the primary winding 13. The bridge circuit components may be selected so that the ratio between bridge leg 26 and the bridge leg 27, as well as the ratio between the opposite bridge legs, is in the ratio of 25:6, and the thermistor 31} maybe of the bar or disk type having a tem perature 'coetlicient of about 3 or 4 percent per degree centigrade.

A second embodiment of a condition sensitive control circuit constructed in accordance with the present invention, is illustrated in FIGURE 4 of the drawings. This second embodiment of the invention utilizes a transistor 51 having a base electrode 52, a collector electrode 53, and an emitter electrode 54. The base electrode 52 is connected through a biasing resistor $5 to a voltage dividing network formed by a pair of resistors 56 and 57 connected in series across a pair of output terminals of a direct current voltage supply source (not shown). The emitter electrode 54 of transistor 51 is connected directly to the positive terminal of the direct current voltage supply and the collector electrode of transistor 51 is connected through a winding 58 of a transformer 59 to the negative terminal of the source of direct current supply voltage. The primary winding 58 of transformer 59 is inductively coupled to a secondary winding 61 which is included in a feedback bridge network. The feedback bridge network is comprised by the two halfs of the secondary winding 61a and 61b connected in a closed loop with a variable resistor 62 and a thermistor 63 or other condition sensitive element whose electrical resistance varies in accordance with the variations in the condition to be sensed. The midtap point of the winding 61 is connected through a capacitor 64 to the base electrode 52 of transistor 51, and the juncture of the variable resistor 62 and thermistor 63 is connected through a suitable conductor 65 to the emitter electrode 54 of transistor 51 and to the positive terminal of the source of direct current voltage supply. The bridge comprised by the two halfs of secondary winding .61, variable resistor 62 and thermistor 63 comprise a feedback network connected to the transistor 51 in a manner such that when the gain of the overall circuit is greater than 1, the circuit will oscillate in a fashion similar to that described with relation to the embodiment of the invention shown in FIGURE 1. Conversely, when the gain of the circuit is less than 1, the circuit will not oscillate, and current will be drawn by the transistor 51 through the primary winding 58 of transformer 59. Transformer 59 may comprise one part of a relay whose armature (not shown) is connected to a switch or other control device, so that this current may be used to actuate the switch or other control device in response to a change in the condition being sensed by the thermistor 63 or other condition sensitive element.

Assuming the condition sensitive device 63 to be a thermistor, and that the control circuit is used to maintain a predetermined temperature level, the variable resistor 62 may be set at a value such that the potential at the rnidtap point of the winding 61 is equal to the potential of the juncture of the variable resistor 62 and thermistor 63 at the predetermined temperature level. The bias supplied to the base electrode 52 of transistor 51 through resistor 55 is then adjusted so that the gain of the circuit including transistor 51 is equal approximately to 1 when the above condition is satisfied. Thereafter, it the gain of the circuit varies above or below the value -1, as shown in FIGURE 2 of the drawings, the circuit will break into oscillation if its gain becomes greater than 1, or it will fail to oscillate and draw a relatively large current through relay winding 58 when the gain is less than 1. Hence, upon a variation in the temperature from the preset condition, the feedback bridge network will produce an imbalance potential that is fed back to the base electrode 52 through the capacitor 64 to increase or decrease the gain of the circuit depending upon the direction of the temperature change. In the event that temperature increases, then the feedback potential is adjusted to raise the gain of the circuit in a direction to cause the circuit to oscillate thereby allowing the relay associated with the winding 58 to drop out. In the event that the temperature decreases below the preset value, the polarity of the imbalance potential fed back through capacitor 64 drives the circuit in a direction to decrease the gain below 1 so that the circuit does not break into oscillation, and current is drawn through the relay winding 58 of suflicient magnitude to cause the relay to close the contacts thereby actuating the control mechanism controlled by the circuit. It can therefore be appreciated that the present invention provides a new and improved control circuit requiring only a single amplifying device, such as a transistor, and yet provides a snap action control which is extremely sensitive to changes in the condition being sensed. The control circuit is reliable in operation, and because it requires only a single amplifying device, is relatively economic to produce.

Having described several embodiments of a control circuit constructed in accordance with the present invention it is believed obvious that other modifications and variations of the invention are possible in the light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention described which are within the full intended scope of the invention as defined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electrical condition sensing device whose output is indicative as to whether a sensed condition is above or below a preselected condition, the combination of a transistorized relaxation oscillating means comprising a single transistor biased to operate at a point on its gain versus collector current characteristic where the gain goes from a value greater than unity to a value less than unity with increasing collector current whereby depending upon which side of the gain equal unity value the gain of the oscillating means is set, the oscillating means has either an oscillating state or a non-oscillating state, said oscillating means further comprising a switching mechanism including an actuating winding connected in feedback relationship with respect to said oscillating means whereby said actuating winding draws a relatively large actuating current when said oscillating means is in a non-oscillating state and relatively small current incapable of actuating said switching mechanism when said means is in an oscillating state, a voltage divider network connected across a source of constant potential, said network including a condition responsive impedance, circuit means including a bias resistor for connecting the output voltage of said network to said transistor whereby variation in said output voltage resulting from changes in said impedance is effective to determine the state of said oscillating means.

2. In an electrical temperature sensing device whose output is indicative as to whether a sensed temperature is above or below a preselected temperature, the combination of a relaxation oscillating means comprising a single transistor amplifying device biased to operate at a point on its gain versus collector current characteristic where the gain goes from a value greater than unity to a value less than unity with increasing collector current whereby depending upon which side of the gain equal unity value the gain of the oscillating means is set, the oscillating means has either an oscillating state or a non-oscillating state, said osciliating means further comprising a switching mechanism including an actuating winding connected in feedback relationship with re.- spect to the oscillating means and drawing a relatively large current suificient to operate said switching rnechanism when said oscillating means is in a non-oscillating state and drawing a relatively smallcurrent insuificient to operate said switching mechanism when said means is in an oscillating state, a feedback bridge network having two of the four legs thereof formed by the two halves of a winding inductively coupled to said actuating winding whereby a feedback potential is developed across two opposite leg junctions, circuit means for applying a feedback network output signal from the other two leg junctions from said feedback bridge network back to said transistor for controlling the state of operation of said oscillating means, one of said bridge legs including a temperature responsive impedance, said impedance varying said output signal in response to sensed temperature for controlling the state of operation of said oscillating means.

3. In an electricaltemperature sensingv device whose output is indicative as to the direction of deviation of a sensed temperature from a preselected temperature, the combination of a transistorized blocking oscillator means including a single transistor amplifying device biased to operate at a point on its gain versus collector current characteristic where the gain goes from a value greater than unity to a value less than unity with increasing collector current whereby depending upon which side of the gain equal unity value the gain of the oscillating means is set, the oscillating means has either an oscillating state or a non-oscillating state, said oscillator means further comprising a switching mechanism including an actuating winding, with the actuating Winding being connected in the output current circuit of the transistor and being energized when said means is in a non-oscillating state and substantially de-energized when said means is in an oscillating state, a bridge network having four legs, means for applying potential across two opposite leg junctions, circuit means for applying a network output signal from the other two leg junctions to said transistor so as to control the state of operation of said oscillating means, one of said bridge legs including a temperature selection variable resistance having a resistance propontional to said preselected temperature, and another of said legs including a thermistor, the resistance change of said thermistor upon a deviation of sensed temperature from the preselected temperature unbalancing said bridge network, said output signal varying in accordance with said unbalancing so as to control the state of said oscillating means in accordance to the direction of temperature deviation.

References Cited in the file of this patent UNITED STATES PATENTS 2,189,462 Donle et a1. Feb. 6, 1940 2,584,728 Michel Feb; 5, 1952 2,764,643 Sulzer Sept. 25, 1956 2,773,220 Aron Dec. 4, 1956 2,774,919 Coles Dec. 18, 1956 2,955,213 Schaeve Oct. 4, 1960 FOREIGN PATENTS 349,496 Great Britain May 22, 1931 

